Most of silicon wafers widely used as substrates to fabricate semiconductor devices have been manufactured from a silicon single crystal grown by a Czochralski (CZ) method. In the silicon single crystal grown by the CZ method, interstitial oxygen is contained at a concentration of about 1018 atoms/cm3 as an impurity. The interstitial oxygen precipitates due to supersaturation in the course of a thermal history from solidification in crystal growth till cooling to room temperature (hereinafter also abbreviated to a crystal thermal history) and in a heat treatment of a fabrication process for a semiconductor device, thereby a precipitate of silicon oxide (hereinafter also referred to as an oxide precipitate or simply a precipitate) being formed.
The oxide precipitate, works effectively as a site where a heavy metal impurity mixed into a crystal during a device process is captured (Internal Gettering: IG) to improve device characteristics and a product yield. Therefore, an IG capability is attached importance to as one of properties of a silicon wafer.
A process of oxygen precipitation consists of formation of a precipitation nucleus and its growth. Usually formation of a precipitation nucleus progresses in a crystal thermal history and thereafter the precipitation nucleus grows large in a heat treatment such as a device process to be detected as an oxide precipitate. Hence a precipitate formed in a crystal thermal history is herein called a Grown-in precipitation nucleus. As a matter of course, a case arises where an oxygen precipitation nucleus is also formed in a subsequent heat treatment.
In the case of a usual as-grown wafer, an oxygen precipitation nucleus present at a stage prior to a device process is very small in size and therefore has no IG capability. However, through the device process, the oxygen precipitation nucleus grows to a large oxide precipitate having an IG capability.
In order to make a device fabricating region in the vicinity of a surface of the wafer defect-free, a case exists where there is used an epitaxial wafer obtained by depositing silicon single crystals on a substrate in vapor phase growth. In this epitaxial wafer as well, it is important to impart an IG capability to the substrate.
Since an epitaxial step is carried out at a high temperature of about 1100° C. or higher, almost all of oxygen precipitation nuclei (Grown-in precipitation nuclei) formed in a crystal thermal history are annihilated with the result that no oxygen precipitate is formed in a subsequent device process. Therefore, there has remained a problem to reduce an IG capability of an epitaxial wafer.
As a resort to solve this problem, a process has been available in which a substrate prior to an epitaxial step is subjected to a heat treatment at a temperature of the order of 800° C. to thereby grow Grown-in precipitation nuclei formed in a crystal thermal history to sizes which are not annihilated even in an epitaxial step at high temperature. In this process, in a case where a temperature in a heat treatment prior to epitaxial growth is 800° C., for example, Grown-in precipitation nuclei with sizes equal to or larger than a critical size at 800° C. (the minimum size of a precipitation nucleus capable of stable growth at the temperature) grow and survive in the epitaxial step, and further grow in a heat treatment such as a device process following the epitaxial step into oxide precipitates.
Surface distribution in density of Grown-in precipitation nuclei in a substrate prior to an epitaxial step is not necessarily uniform. As a typical example, a case arises where there exist, in the form of a ring, Grown-in precipitation nuclei with comparatively large sizes serving as nuclei of oxidation induced stacking faults (hereinafter also referred to as OSF) to be generated after a heat treatment at a temperature equal to or higher than about 1100° C. in an oxidizing atmosphere (hereinafter a region where OSF are generated in the form of a ring is also referred to as an OSF ring). If such a substrate is subjected to a heat treatment to grow Grown-in precipitation nuclei and then epitaxial growth is carried out, surface distribution in density of precipitates in an epitaxial wafer becomes non-uniform, leading to a problem of non-uniformity in an IG capability.
It has been known that in a general p+ type substrate doped with boron at a high concentration, an OSF ring occurs more easily under an influence of the doped boron as compared with a p type substrate doped with boron at a low concentration.
Therefore, the problem of non-uniformity of an IG capability in a surface of the substrate as described above easily arises especially in a p/p+ type epitaxial wafer using a p+ type substrate. Needless to say, there are some cases where an OSF ring occurs, in addition to a p+ type substrate, on a p type substrate, an n type substrate doped with phosphorus at a low concentration and an n+ type substrate doped with antimony or arsenic at a high concentration, all being used as a substrate for an epitaxial wafer, according to crystal growth conditions. In such cases, a problem of non-uniformity in an IG capability arises as in the case of a p+ type substrate.
A device process in recent years has progressed toward lower temperature and shorter time with the use of a larger diameter wafer. For example, a series of device processes are all carried out at a temperature of 1000° C. or lower, and RTP (Rapid Thermal Processing) is frequently used that requires a heat treatment only for a time of the order of tens of seconds. In such a device process, since there is a case where the sum of the total heat treatments corresponds to only a heat treatment of the order of 2 hours at 1000° C., there cannot be expected growth of oxide precipitates during the device process, which was realized in the prior practice. For such a reason, in a device process carried out at low temperature for a short time, a necessity arises for imparting an excellent IG capability to a wafer prior to a device process.